1. Field of the Invention
The present invention relates to an airbag device for a driver's seat mountable on a steering wheel of a vehicle. In particular, the invention relates to an airbag device in which a vent hole of an airbag covered by a cover portion, which is continuous with a tether that controls a clearance between a vehicle-side wall and a driver-side wall of the airbag at deployment, is openable when the cover portion loosens along with loosening of the tether when the airbag cushions the driver.
2. Description of Related Art
JP 2014-046724 A, by way of example, discloses a known airbag device for a driver's seat including an airbag which is stored in a boss section disposed at a generally center of a rim of a steering wheel in a folded-up configuration. The airbag is inflatable in such a manner as to separate a vehicle-side wall and a driver-side wall when fed with an inflation gas. The airbag includes more than one tether which connect the vehicle-side wall and the driver-side wall for controlling a clearance between the vehicle-side wall and the driver-side wall at airbag inflation, an inlet opening disposed at the generally center of the vehicle-side wall for introducing an inflation gas and a vent hole located in a vicinity of the inlet opening on the vehicle-side wall for exhausting the inflation gas. The vent hole is composed of an outer opening formed on an outer face region of the vehicle-side wall and an inner opening formed on an inner face region of the vehicle-side wall in such a manner as to overlap with the outer opening. The inner face region is arranged on an inner side of the outer face region. One of the tethers serves as an exhaust strap. The exhaust strap includes a cover section which is held between the outer face region and inner face region for covering the outer opening and a pair of connecting bands extending from opposite sides of the cover section and connected to the driver-side wall. The outer face region and inner face region are joined together by an inner edge close to the inlet opening and an outer edge facing away from the inlet opening while holding the cover section between the outer face region and inner face region. Accordingly, the connecting bands extend from the cover section in a symmetrical fashion with respect to a straight line connecting the inlet opening and vent hole as viewed from the driver-side wall at airbag deployment. Further, the cover section is joined to the vehicle-side wall by its edge proximate the inlet opening.
If the airbag as deployed cushions the driver, the driver-side wall moves towards the vehicle-side wall and the connecting bands loosen. Then the edge of the cover section not joined to the vehicle-side wall slips out of the outer opening and is deployed out of the outer face region, which provides gas communication between the outer opening and inner opening and opens the vent hole.
In the above conventional airbag device, the exhaust strap is joined to a position on the driver-side wall slid from the vent hole along the steering shaft to which the steering wheel is mounted, not to the center of the driver-side wall. With this configuration, the loosening amount of the connecting bands of the exhaust strap as the airbag cushions the driver is likely to vary from an instance where the vehicle is driven straight ahead to an instance where the steering wheel is half turned from the straight-ahead position. That is, the steering shaft is mounted at a slant and the steering wheel is mounted on the steering shaft such that the rim forms a right angle to the steering shaft. If, in this configuration, a vent hole is disposed in a front area of the vehicle-side wall (i.e., at a distance from the driver) when the rim is at the straight-ahead position, it will be located in a rear region of the vehicle-side wall (i.e., closer to the driver) when a half turn is made. If the airbag cushions the driver when the steering wheel is at the half turned position, the loosening amount of the connecting bands of the exhaust strap will be greater than that at the straight-ahead position. That is, an amount of an inflation gas released from the vent hole may vary depending on the angular position of the steering wheel, which gives a room for improvement.